English

Polar alignment of a circumbinary disc around a brown dwarf binary

Earth and Planetary Astrophysics 2025-07-01 v1 Solar and Stellar Astrophysics

Abstract

Inspired by recent observations suggesting that the retrograde precession of the brown dwarf binary 2M1510 AB is consistent with induction by a polar circumbinary planet, we investigate the formation of such planets by studying the evolution of a primordial misaligned circumbinary disc around a brown dwarf binary. Analytical calculations show that a critical tilt angle of icrit50i_{\rm crit} \gtrsim 50^\circ for moderately eccentric binaries is needed for polar alignment of circumbinary discs in systems with low disc-to-binary angular momentum ratios. For higher ratios, this angle converges to the Kozai-Lidov instability threshold of 39\sim 39^\circ. We identify disc parameters, such as viscosity (α=104\alpha = 10^{-4}) and aspect ratio (H/r=0.05H/r = 0.05), that enable polar alignment within typical disc lifetimes. Notably, a circumbinary disc around a low-mass binary, such as a brown dwarf binary, will require more time to achieve polar alignment compared to higher-mass systems. A hydrodynamical simulation confirms that an initially inclined disc around a brown dwarf evolves towards a polar state, creating favorable conditions for polar planet formation. Using these results, we finish by placing 2M1510 AB into a wider context and speculate why such a polar circumbinary configuration has not been identified before.

Keywords

Cite

@article{arxiv.2506.22747,
  title  = {Polar alignment of a circumbinary disc around a brown dwarf binary},
  author = {Jeremy L. Smallwood and Thomas A. Baycroft and Amaury H. M. J. Triaud and Richard P. Nelson},
  journal= {arXiv preprint arXiv:2506.22747},
  year   = {2025}
}

Comments

9 pages, 7 figures, Accepted for publication in MNRAS Letters

R2 v1 2026-07-01T03:37:34.178Z